During the build-up leading to a serious accident in a nuclear power plant chemical processes of various natures cause hydrogen to be produced. This can lead to the formation of flammable gas mixtures in the reactor containment. If hydrogen is released and concentrated over a longer period of time, mixtures capable of detonation can be formed. This means that the integrity of the reactor containment, the last barrier for the retention of fission products, will be jeopardized. (The term "reactor containment" is used here as a generic term for all compartments in which the problem described may arise and must thus be solved).
Known in the art are measures for the prevention of the danger arising from such a flammable gas mixture that are aimed at eliminating the hydrogen in the compartments of a reactor containment. These measures include the use of igniters, as well as the catalytic recombination into water of the hydrogen with the oxygen present in the reactor containment (e.g., EP-A-0 303 144). Especially promising is the use of catalytic recombiners, which meanwhile have become known in the art in various designs (EP-A-0416 143, DE-A-36 04 416, EP-A-0 303 144, DE-A-40 03 833), although these are not fully capable of eliminating the danger of a detonation or even a deflagration, for reasons that will be explained in the following.
Depending on the steam content of the atmosphere within the compartments of a reactor containment, the deflagration limit may be reached even with a local concentration of hydrogen of as little as 4%. It is a known fact that steam has an inerting property, which is to say that with a higher steam content the deflagration limit is not reached until higher concentrations of hydrogen are generated. (The term "inerting," which is translated from the German word "Inertisierung," is used herein to mean decreasing the danger of explosion of an explosive gas mixture by reducing the concentration of explosive components in the gas mixture.) From model tests it is known that at the beginning of a nuclear core melt-down accident steam is released first while hydrogen is not released until after a certain delay. The composition of the gas mixtures in the different compartments of a reactor containment can, however, vary from one another very extensively and can change continuously during the further progression of the accident.
The reaction speed of the catalytic recombiners (catalysts) increases exponentially with the temperature. The catalysts heat up until an equilibrium is reached between the heat that is produced and the heat that is carried off. It is only after higher catalyst temperatures have been reached that the reduction of hydrogen will accelerate and the convection resulting from the increase in temperature will cause mixing of the surrounding atmosphere.
If the supply of hydrogen within a given compartment proceeds faster than it is eliminated, an increased hydrogen concentration will result within the gas mixture. The steam content, which at first will not necessarily be equal in all compartments of the reactor containment, will be reduced during the continued course of the process by condensation at the cold walls, thereby reducing its inerting effect.
The so-called detonation cell size constitutes a measure for the propagation of a detonation as well as for the sensitivity of a gas mixture to detonation. The smaller the cell size, the greater will be the susceptibility of the gas mixture to detonation. It is known that dilution of the gas mixture containing hydrogen by the use of steam and even more by CO.sub.2 causes an increase in the detonation cell size. This is true for both lower and higher temperatures. In a gas mixture at 100.degree. C. with a stoichiometric composition, the detonation cell size will be increased fivefold or 34-fold by the addition of 10% or 20% by volume of CO.sub.2, respectively (fourfold or sixfold in the case of steam), compared to that without the addition of CO.sub.2 (or steam). Nothing has been demonstrated so far about what the effect would be of diluting the gas mixture simultaneously with steam and CO.sub.2. It may be assumed, however, that the effect would be at least additive.
The detonation cell size of a gas mixture of like composition will be reduced through an increase in temperature and pressure. During an accident situation a temperature of around 100.degree. C. will prevail in the compartments of the reactor containment. Opposing this, a significantly lower temperature in the gas mixture can result in the immediate vicinity of a cold concrete wall. This will cause an increase in the detonation cell size. However, the detonation cell size of the gas mixture will potentially tend to decrease at the same time because of a reduction in the steam content owing to condensation.
Consideration has been given to the possibility of making use of the inerting effect of CO.sub.2 to prevent the danger of detonation during an accident situation in a reactor containment. In conjunction with this, a distinction has been made of a so-called pre-inerting and a so-called post-inerting. In pre-inerting the compartments of the reactor containment of the nuclear power plant are filled with nitrogen (N.sub.2) so that when an accident begins to occur, no oxygen would be available to form a flammable gas mixture with the hydrogen that would then be produced. But such a type of pre-inerting involves such practical problems that no actual significance attaches to it. It is sufficient to merely mention that problems would arise with accessing a reactor containment containing a pure nitrogen atmosphere during normal operation.
By post-inerting is meant an injection of liquid CO.sub.2 into the reactor containment that is triggered only at the onset of an accident. This post-inerting represents an active safety measure and for this reason in itself is not very realistic. The word "active" means that some sort of device has to be present which senses the fact that an accident has occurred and which activates the introduction of CO.sub.2. Every type of active measure suffers from the fact that it cannot be relied on one hundred per cent to function properly in an emergency. In addition, serious problems arise from feeding in cold CO.sub.2 of -78.degree. C. Feeding in this cold gas would cause a drastically increased condensation of the steam present in the reactor containment and cancel its inerting effect. In addition, this injection would of necessity lead to a subsequent increase in pressure, which, as explained above, would reduce the detonation cell size. Finally, it is very uncertain what the effect would be of the low temperatures involved in supplying cold gas on the relevant safety devices in the reactor containment.
The catalytic recombiners, which are passive safety devices, represent mechanisms contributing considerably to reduce the risks involved in an accident situation as described, but they do not eliminate the danger. The possibilities of both pre-inerting and post-inerting do not appear to be practicable.